A polyimide-metal foil composite laminate has been employed in semi-conductor devices as a base, of flexible printed circuit boards and the like. The flexible printed circuit board to which electronic parts, etc. are fixed is mounted in a semi-conductor device, etc. The recent demand for increasing a mounting density of a semi-conductor device has been satisfied by fixing electronic parts to the polyimide-metal foil composite laminate having a printed circuit at a high density by soldering on plural sites. Since the soldering is performed by melting a solder at a considerably high temperature, the polyimide-metal foil composite laminate to be used as a base is requested to have excellent characteristics including heat resistance. It is also requested from the standpoint of operation that the composite laminate remains flat without suffering warping during the stage not only before but after the circuit formation.
The above-described polyimide-metal foil composite laminate has been produced by (1) a process comprising adhering a polyimide film to a metal foil by using an adhesive, (2) a process comprising heat-fusing a polyimide film to a metal foil, and (3) a process comprising applying a solution of a polyimide or a precursor thereof in an organic polar solvent to a metal foil and heating the ,coating to form a polyimide film.
The first process using an adhesive is disadvantageous in that the overall heat resistance or heat shock resistance of the resulting composite laminate is reduced due to the action of the adhesive. In addition,. this process requires previous film formation of polyimide, making the process time-consuming.
The second process is freed from the problems associated with the use of an adhesive as in the first process but still requires filming of polyimide in advance.
The third process involves no problem as described above and has an additional advantage that a polyimide-metal foil composite laminate having a small thickness can be obtained. However, this technique is disadvantageous in that the composite laminate either before or after etching of the metal foil, e.g., a copper foil, undergoes curling, which results in serious hinderance to workability. Such curling is believed to arise from the following causes. The curling occurring before etching is ascribed to a difference in coefficient of thermal expansion between a polyimide resin layer and a copper foil. That is, the difference in coefficient of thermal expansion produces a difference in shrinkage during the time after completion of heat-imidation through cooling to room temperature, thus causing curling. On the other hand, the curling after etching of the copper foil is ascribable to a difference in residual stress inside the polyimide resin layer. That is, during the imidation of the polyamide acid solution applied on the copper foil, one side of the resin layer comprising the polyamide acid is fixed to the copper foil with the other side open to air. As a result, there is produced a difference in the degree of drying and imidation between the side in contact with the copper foil and the opposite side in contact with air, which causes a difference in residual stress during cooling to room temperature. While the polyimide resin layer is supported by the copper foil in a composite structure, the residual stress, though surely existing, is compensated by rigidity possessed by the copper foil so that no curling takes place. However, the residual stress comes to cause curling upon removal of the copper foil by etching.
In order to solve the curling problem of a flexible printing base comprising the polyimide-metal foil composite laminate, it has been proposed to correct the curl by subjecting the composite laminate to post-treatment. For example, JP-A-54-31480 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application") discloses a process in which the composite laminate is stretched and rolled in an arched drier; JP-A-54-66966 discloses a process in which the composite laminate is heat-treated to remove the curl; JP-A-54-108272 discloses a process in which the composite laminate is wound around a cylinder with the resin layer facing outside and heat-treated at 100 to 200.degree. C. for a long time to remove curling; and JP-A-59-22388 discloses a process in which the metal foil of the composite laminate is continuously brought into contact with a bar whose surface has a curvature radius of from 0.5 to 25 mm under tension to remove curling. These techniques fairly achieve correction of the curling but still present problems on practical application because an additional apparatus for the respective post-treatment is required, involving a so much extended processing step.
It has also been proposed to prevent curling by reacting a heat resistant resin with an epoxy resin, a phenoxy resin, or an acrylonitrile-butadiene copolymer to form a three-dimentional structure which is inhibited from shrinking on curing. According to this technique, prevention of curling can be achieved to some extent. However, the resin having a three-dimensional structure lacks flexibility and suffers from reduction of heat resistance due to a crosslinking agent introduced so that the resulting composite laminate exhibits deteriorated performance properties.
In recent years, an aromatic polyimide having a coefficient of thermal expension equal to or less than that of a copper foil has been developed. It has been confirmed that a flexible printing base (composite laminate) composed of such a polyimide layer and a metal foil undergoes substantially no curling as described in JP-A-60-32827. This flexible printing base, however, still undergoes curling in the state where the copper foil has been removed by etching. This is due to the difference in residual strain between the polyimide resin layer in contact with the copper foil and the other side. In other words, polyimide having such a small coefficient of thermal expansion as exhibited by a copper foil generally has a rigid molecular chain so that the internal stress is liable to remain without being relaxed.
The above-described proposals concern prevention of curling of a printing base having a metal foil. With respect to prevention of curling of a printing base from which the metal foil has been removed by etching, JP-A-5815579 discloses a process comprising casting a polyimide solution prepared by using a tetranuclear diamine on a metal foil. The flexible printing base prepared by this process does not suffer from curling even after the metal foil is etched off. This seems to be because the molecular chain of the polyimide has a folded structure, and the internal stress generated in the polyimide resin layer is properly relaxed by this folded structure and thus hardly remains. However, since polyimide having such a structure has a large coefficient of thermal expansion, it causes considerably large curling in the state where it is laminated with a copper foil.
As described above, any of the conventionally proposed processes has its own advantages and disadvantages, and the flexible printing base obtained undergoes curling either in the state of a composite laminate or after etching of the metal foil, leading to disorders in photoresist printing and the like as well as problems of handling, such as transportation.
Other recent proposals for overcoming these problems include a process in which an aromatic polyimide layer having, in the molecule thereof, a constituting unit for decreasing the coefficient of thermal expansion to the level of a copper foil and a constituting unit for relaxing the internal stress is formed on a copper foil, as disclosed in JP-A-58-190093 and JP-A-60-206639. However, the flexible printing base produced by this process still undergoes curling in any of the stage of a composite laminate and the stage after etching.
Hence, a flexible printing base which does not curl at all in any stage and also exhibits excellent characteristics, such as heat resistance, chemical resistance, dimensional stability, etc., has not yet been developed.